322
THE SEAS.
method, it is necessary now to put to “sea to these grounds.
The plaice egg we have said is pelagic and drifts about in
the water layers above the bottom.
To catch these eggs
therefore a tow-net must be used. '
The area to be searched
is divided up on the chart into a number of small sections
in each of which a point or “ station ” is ,marked, at which
hauls with the tow-net are to be made.
Generally, when a
research vessel is on a “ plaice-egg cruise,” a vertical
tow—net is used ; that is, a net which is hauled up vertically
through the water from bottom to surface.
The depths
being known, it thus becomes possible to compare with
accuracy the actual numbers of eggs caught in different
,
localities.- At the end of the cruise the numbers of eggs
taken at each station are marked out on the chart and
-
bontours drawn throughthe stations where approximately
'
the same numbers of eggs were -caught (Fig. 61). In
this way, if the whole area in which the eggs were actually
present has been studied, a map should be produced showing
,
the largest numbers "of -eggs caùght near a centre, the
numbers becoming less and less the further one goes from
this centre.
Now the-worker, who was counting the eggs,
at the same time made notes as to the stage of development”
of the young
sh within the
By the use of drift
bottles and current metres, as described in a previous
'
a rough measure can be obtained of the move-.
,
ments of the water in the regions'examined.
It has been
Workedout how long it takes a plaice‘egg to develop under
different temperature conditions, and the temperature
of the—water being known the investigator can immediately
reckon how many days have elapsed
the actual
spawning of the eggs which he has taken.
KnoWing also
the rate and direction of the water movements he Can lay
_
_
hisnger on the spawning grounds.
_
,
Other methods need ,to”-be resorted to in a search for the
_—